1. Field of the Invention
The field of the invention is controller output circuits, and particularly, those included in circuit modules used in programmable controller I/O interface racks such as that disclosed in Struger, et al., U.S. Pat. No. 4,151,580, issued Apr. 24, 1979.
2. Description of the Prior Art
A controller output circuit for a prior controller is disclosed in Struger, et al., U.S. Pat. No. 3,745,546, issued July 10, 1973. There an output drive circuit for a programmable controller includes a triac (a bidirectional triode thyristor) connected in series with an a-c power supply, a fuse and a controlled external load. The fuse is shunted by a neon indicator lamp and the triac is shunted by a high impedence path through a resistor and a capacitor. When the fuse blows the triac is turned off and current flows through the neon bulb to indicate a fault.
There are several possible sources of faults in such output circuits, and it is desirable to pinpoint the source of such faults to prevent the repetitive blowing of fuses or the degradation of the output devices controlled by the output circuits. Where a triac is included in the output drive circuit, it is desirable to know whether the triac itself has failed.